Generally, the molding of rubber articles, particularly tire carcasses, requires that a composition be applied on either the sidewall or the entire outside of the uncured article before it is placed into a mold for cure. While such compositions facilitate mold release and improve molding characteristics, their main function is to act as an air venting medium for entrapped air and thus reduce the blemishes which can form on the outside surface during curing. For example, if air is trapped between the wall of a tire and the mold during curing and shaping, an air bubble blemish is cured into the tire. The air bubble appears as a depression in the tire sidewall.
Currently, coatings which provide high adhesion performance together with adequate venting when applied to rubber articles prior to molding and curing are based on low flash hydrocarbon solvents containing as little as 5-10% solids. Such low flash, high volume solvent compositions release a large amount of solvent to the atmosphere during application (usually by spraying) and vulcanization. Objections have been raised by OSHA (Occupational Safety and Health Act) against such solvent based products due to the hazardous atmosphere created during application, leading to labor problems relating to the handling of such low flash, high volume solvent products. Such compositions also create a fire hazard.
Moreover, low boiling solvent coating compositions currently used contain 5-10% carbon black in suspension along with a substantially lesser amount of dissolved rubber. The solvent in such compositions evaporates quickly when the composition is sprayed on the outside surface of an uncured tire, leaving a thin surface film of carbon black and rubber that becomes tightly bound to the rubber after curing or vulcanization. In such solvent-based compositions, the high carbon black loading needed to improve air venting often causes inadequate adhesion. This is particularly troublesome because good adhesion is critical for tire substrates having tread over sidewall type construction.
Another drawback of solvent based compositions is that they are typically not stable on storage and must be used within a relatively short period of time after being prepared. For this reason, they are typically made in-house by the user.
On the other hand, while water based compositions currently on the market containing both carbon black and mineral fillers together with rubber latex binders provide adequate air venting, they have almost no adhesive properties. Water based compositions tailored to provide adequate adhesion do not provide adequate air bleed characteristics.
In order to provide a coating for rubber articles devoid of the disadvantages of prior coatings, a water based composition that is compatible with and effectively adherent to rubber surfaces while permitting relatively short drying times with adequate air bleed characteristics on curing is desirable. Such a coating must eliminate blemishes such as folds, unfilled voids, knit failures and cracks on the surface without flaking or leaving a residue on mold surfaces. They must have adequate storage and handling stability and have no deleterious effect on the weatherability and aging characteristics of the cured rubber article.